Sea level-ice sheet dynamics may help stabilize Antarctica’s ice

What happens if the West Antarctic Ice Sheet collapses? It turns out that …

To say that there is uncertainty in future sea level rise is an understatement. If you combine the contribution from Greenland, Antarctica, and mid-latitude glaciers with the expected thermal expansion of the ocean, estimates of total sea level rise by the year 2100 range from 0.2 to 2 meters.

Two meters of sea level rise would put much of the Netherlands, New Orleans, Bangkok, and other heavily populated areas underwater. Even coastal areas that are not under water are likely to see coastal erosion and similar problems increase with a two-meter rise in sea level.

Clearly this is an important subject, but where does an order of magnitude of uncertainty come from? One of the least understood aspects is the role of glacial dynamics. Just how does an ice sheet move? Part of it is "simple" deformation of the ice itself. Here, you can think of the bottom of the ice sheet as being frozen to the bed, the next layer of ice up is sliding very slowly over that layer, and each successive layer slides a little faster than the layer below it. But there's that "frozen to the bed" thing. What if the ice isn't frozen to the bed?

An ice sheet that is not stuck to the bed, but instead lubricated by a layer of water underneath, can slide much faster; this is part of what leads to the uncertainty in projected sea level rise. In particular, the West Antarctic Ice Sheet has the potential to be practically lifted off its bed by the very same rising sea level it is helping create. This would lead to it slide faster, raising sea level, which in turn causes it to rise faster still. If the entire ice sheet were lost, it could increase sea level by almost five meters.

This brings us to a recent paper in Nature Geoscience. The paper, by Natalya Gomez and co-authors, looked at several effects on ice sheet dynamics that have previously been ignored.

The first is isostatic rebound. Basically, the mass of ice on Antarctica is so great, it has actually caused the continent itself to sink into the underlying mantle. You can think of the continents as giant ice cubes floating on the mantle: the larger they are, the deeper they sit. But when you remove a lot of mass from the top of the continent—let's just say for example, an ice sheet—the continent will start to rise back up from the mantle, thus counteracting some of the local rise in sea level. However, the timescales for isostatic rebound are very long. For example, North America is still rebounding from the loss of its ice sheets around ten thousand years ago.

In addition, the ice sheet has enough mass to exert its own gravitational pull. That pull is in fact drawing the ocean towards it, so removing the ice sheet will also lead to a decrease in local sea level relative to global sea level.

Finally, the authors look at the effect of moving all that mass on the spin of the Earth itself. Think of ice skaters moving their arms further from their body while they are spinning. The ice at the pole is closer to the axis of rotation, so as it melts and is redistributed towards the equator, the Earth will slow down ever so slightly. In yet another twist, if the earth's rotation decreases, the distribution of ocean water changes. In effect, the Earth is acting as a giant centrifuge, spinning the oceans outwards, and thus towards the equator. So, if you decrease the earth's rotation, you increase sea level at the poles.

With all of these competing factors, the authors had to use a simple model to calculate the interactions and thus the relative stability of an ice sheet with rising sea level. Previous theories had suggested that any time an ice sheet was resting on a "reversed bed slope" (i.e. the land surface is lower inland and higher at the coast), the ice sheet would be unstable with increasing sea level. However, by accounting for the other factors that play into local sea level, the authors of the present study suggest that, even on reverse bed slopes, the changes in sea level will actual result in a stabilizing effect on the ice sheet under most conditions.

Unfortunately, at the end of the day, we still do not have much more certainty about future changes in sea level, but at least now we think we are less likely to see a catastrophic collapse of the West Antarctic Ice Sheet. That is until the next study appears on how the ice sheet will be self-lubricated by its own meltwater.